To an increasing extent, preparation chambers in which both the specimens and the tools are cooled to low temperatures and kept there (so-called cryopreparation) are being used for microscopic and electron-microscopic investigations, particularly of biological specimens. In this connection, neither the specimens nor the tools can be allowed to become frozen over with condensation from the moisture in the air. Liquid nitrogen is used as the coolant in most instances.
Published German patent application DE-OS No. 29 06 153 discloses a cooling chamber in which the work chamber is a block-like vessel made of sheet metal having a good thermal conductivity. This work chamber is provided with feet by which it is secured in a larger coolant vessel, also of block-like configuration, which receives the coolant and, in turn, is mounted in an insulating vessel made of foamed plastic. The insulating vessel is surrounded by a metal housing. The vaporized coolant in the coolant vessel is conducted away through conduits between the insulating vessel and the metal housing. To prevent the outside of the metal vessel from freezing over with moisture from the air, an electric heater must be used to heat this vessel. Condensation of moisture in the air in the work chamber is avoided by flushing the chamber continuously; to this end, helium gas is cooled and dried by means of a cooling coil mounted in the coolant vessel. The helium gas flows out at the bottom of the work chamber and completely fills the work chamber because the work chamber is made to be tight except for its opening at the top.
From published U.S. Pat. No. 4,302,950, it is further known to use part of the vaporized coolant to flush the work chamber. Sheet-metal panels are provided for this purpose on the walls of the work chamber and guide the vaporized coolant to the bottom of the work chamber. U.S. Pat. No. 4,302,950 also discloses an electric heater disposed in the coolant to adjust the quantity of coolant that is vaporized for flushing the work chamber. In this embodiment as well, it is necessary to heat the metal housing to avoid condensation of the moisture in the air.
These known cooling chambers have the disadvantage of requiring relatively large quantities of coolant. In the case of liquid nitrogen, which is used most often, approximately 30 liters are consumed in 5 hours. By present standards, this makes the running costs for routine operation too high.